为什么JDK 9中把String的char[]改成了byte[]？

/* The value is used for character storage. */
private final char value[];

/**
 * The value is used for character storage.
 *
 * @implNote This field is trusted by the VM, and is a subject to
 * constant folding if String instance is constant. Overwriting this
 * field after construction will cause problems.
 *
 * Additionally, it is marked with {@link Stable} to trust the contents
 * of the array. No other facility in JDK provides this functionality (yet).
 * {@link Stable} is safe here, because value is never null.
 */
@Stable
private final byte[] value;

/**
 * The identifier of the encoding used to encode the bytes in
 * {@code value}. The supported values in this implementation are
 *
 * LATIN1
 * UTF16
 *
 * @implNote This field is trusted by the VM, and is a subject to
 * constant folding if String instance is constant. Overwriting this
 * field after construction will cause problems.
 */
private final byte coder;

static final byte LATIN1 = 0;
static final byte UTF16 = 1;

public int indexOf(int ch, int fromIndex) {
    return isLatin1()
        ? StringLatin1.indexOf(value, ch, fromIndex)
        : StringUTF16.indexOf(value, ch, fromIndex);
}

private boolean isLatin1() {
    return COMPACT_STRINGS && coder == LATIN1;
}